The protesters say the new rules will catch tens of thousands of recently retired workers with no pension cover. Thousands braved the rain to attend the demonstration, many carrying the red flags of the CGIL, Italy's largest union which organized the march with the more moderate CISL and UIL labor confederations.

The large demonstration underlined wider discontent with Monti's unelected government of technocrats at a time when financial markets are on edge again as worries about Spain's budget problems spill over into other euro zone economies, including Italy.

"I don't hate Monti but unfortunately technocrats are number crunchers and they don't know that behind the numbers are real people," said 58-year-old Antonio Lupano, a retired print worker marching through central Rome.

The government has been striving to impose painful reforms on a debt-burdened economy that has been among the most sluggish in the euro zone for more than a decade.

Labor Minister Elsa Fornero estimates that 65,000 workers who took early retirement will be affected by changes that start raising the minimum pension age for many employees from this year.

Fornero says amendments to the original legislation will ensure that all of those affected have adequate cover, but the numbers are disputed by unions, which say that as many as 300,000 workers may be left to get by with neither job nor pension.

Silvana Garzia, a 57-year-old from Naples who took early retirement from the big communications company Telecom Italia, says she now risks six years with no income at all before she qualifies for a pension.

"I'm absolutely against the Monti government because it's making the poorest pay and no-one else," she said.

HOSTILE

Monti was hailed as the savior of Italy after taking over from scandal-plagued Silvio Berlusconi in November at the height of last year's debt crisis. But since then, the climate around him has become much more hostile.

The parties that back Monti in parliament have become noticeably more critical of the former European Commissioner as local elections, due on May 6-7, approach. Continuing expressions of support are now increasingly hedged with demands as politicians try to recover some support from disaffected voters.

After passing a sweeping mix of tax hikes and spending cuts to shore up public finances, Monti undertook a major overhaul of the pension system, which was largely praised by economists. But he has faced a tougher fight over moves to deregulate the economy and reform rigid labor market rules.

Planned reforms that would overhaul labor market regulation and make it easier for companies to lay off employees are currently going through parliament following concessions made to unions earlier this month.

Those concessions, which would allow courts in some circumstances to order the reinstatement of workers deemed to have been fired without proper justification, have angered employers who complain the change will create more uncertainty.

They leave Monti facing a battle in parliament to satisfy the centre-right and business lobbies, which want to reduce proposed penalties designed to discourage companies from offering temporary contracts instead of permanent jobs.

Monti's popularity ratings have been hit by the standoff over labor market reforms which many feel will cause serious hardship at a time when unemployment is rising and the chronically stagnant economy is sliding further into recession.

His argument that the reforms will help fix a labor market divided between full-time workers on permanent contracts and a growing army of mainly younger people in insecure, dead-end jobs has so far failed to persuade his critics.

(Additional reporting by Francesca Piscioneri, writing By James Mackenzie; Editing by Barry Moody and Alessandra Rizzo)